Power devices are widely used to form various types of switch elements such as power supply switches, telecommunication switches, power switches and the like. They must have an active zone to allow a large amount of current to pass through, and also a termination zone to withstand a greater breakdown voltage.
Hence, in terms of design of the power device, aside from the active zone for current to pass through, a termination structure also has to be provided to avoid breakdown from taking place too soon during reverse bias operation, thereby to improve reliability of the element. The conventional termination structure generally can be divided into several types, such as local oxidation of silicon (LOCOS), field plate and guard ring. U.S. publication No. 2010/0032685 discloses a power device equipped with termination structure that includes a substrate. The substrate contains a drift layer with a first conduction type, a buffer layer with a second conduction type different from the first conduction type, and a termination structure with the second conduction type. The buffer layer is located on the drift layer and forms a P-N junction with the drift layer. The termination structure is located on the drift layer abutting the buffer layer. The buffer layer has a step portion extended to the termination structure to cover partially the termination structure, thereby to withstand the electric field generated by a high breakdown voltage.
The aforesaid technique has to form an extra buffer layer to enhance the capability of the power device to withstand the breakdown voltage. This not only requires additional processes for manufacturing the buffer layer during fabrication of the power device, also leaves room for improvement on the power device in terms of withstanding the breakdown voltage.